1. Field of the Invention
The present invention relates to an internal combustion engine that directly injects fuel into a combustion chamber via a fuel injection valve, and more particularly to techniques for fuel supply to suppress bubble generation in the fuel in a delivery pipe.
2. Description of the Related Art
Known direct injection internal combustion engine directly, which injects fuel into a combustion chamber to cause combustion, is provided with a fuel injection valve to allow injection of fuel into the combustion chamber. The fuel injection valve is attached to a fuel distributing pipe called a delivery pipe to supply highly pressurized fuel. Since the delivery pipe is attached to the internal combustion engine, the fuel inside the delivery pipe is heated by the internal combustion engine in the operation, whereas the fuel is cooled down after the end of the operation. With the cooling down, the fuel inside the delivery pipe contracts, to lower the pressure of the fuel inside the delivery pipe. As a result, the fuel pressure in the delivery pipe drops below a saturated vapor pressure at the temperature drop, to generate bubbles of fuel vapor or of air dissolved in the fuel in the delivery pipe.
At the start-up of the internal combustion engine, the fuel pressure inside the delivery pipe is required to be risen to a certain degree. However, when the bubbles are generated in the fuel in the delivery pipe, the fuel pressure does not rise in the delivery pipe until the bubbles are broken. When the bubbles are formed in the fuel in the delivery pipe, the injection of the fuel from the fuel injection valve cannot start immediately after the start-up. To suppress the bubble generation in the fuel in the delivery pipe, a technique is suggested in Japanese Patent Application Laid-Open No. 2003-120460 to maintain the fuel pressure in the delivery pipe at a level equal to or higher than a set pressure during the suspension of the engine operation thereby suppressing the bubble generation in the fuel in the delivery pipe.
When the internal combustion engine is employed as a power source for a vehicle, a battery is usually mounted on the vehicle to start up the internal combustion engine. The technique disclosed in Japanese Patent Application Laid-Open No. 2003-120460 drives a fuel pressurizing unit for the pressurization of fuel in the delivery pipe in the internal combustion engine under suspension, thereby increasing the power consumption. Though the Japanese Patent Application Laid-Open No. 2003-120460 does not disclose the inconvenience of increased power consumption during the engine suspension, there remains a room of improvement.
In addition, when the fuel pressure in the delivery pipe is excessively high, the fuel may leak out from a fuel injection port of the fuel injection valve (hereinafter referred to as “fuel leakage”). The fuel leakage makes the fuel atomization poor at the start-up of the internal combustion engine, resulting in degraded emission. To eliminate such inconveniences, the fuel pressure in the delivery pipe during the engine suspension is preferably in a range that allows the suppression of bubble generation and maintains the fuel leakage to an acceptable degree. The technique disclosed in Japanese Patent Application Laid-Open No. 2003-120460, though allows the suppression of bubble generation in the fuel in the delivery pipe, cannot eliminate the occurrence of unacceptable fuel leakage depending on the condition of pressurization.